Concatenate the first name and last name of each customer in the customers table, separated by a space. Handle null values by replacing them with an empty string.
CREATE TABLE customers (customer_id varchar, firstname varchar, lastname varchar); INSERT INTO customers (customer_id, firstname, lastname) VALUES ('C001', 'John', 'Doe'),('C002', 'Jane', NULL),('C003', NULL, 'Smith');
```sql
SELECT COALESCE(firstname, '') || ' ' || COALESCE(lastname, '') AS full_name FROM customers;
```
```sql
SELECT COALESCE(firstname, '') || ' ' || COALESCE(lastname, '') AS full_name FROM customers;
```